1. Field of the Invention
The present invention relates to a fluid bearing head for use in information recording and/or retrieval apparatus, and more particularly, to a method and means for supporting an optical read or write head to maintain a constant spacing between the head and a moving information storage medium.
2. Description of the Prior Art
In the design and construction of an optical record and/or playback system, it is necessary to maintain a constant spacing between the read or write head and the recording medium in order to, in turn, maintain proper focus of the light beam impinging the surface of the medium. In a videodisc system, for example, information is recorded on a disc as a series of irregularities, each approximately 0.6 microns in width, and arranged in a spiral track with adjacent tracks being spaced apart by approximately 1.5 microns center to center. As noted in U.S. Pat. No. 3,914,541, issued Oct. 21, 1975 to James E. Elliott, the problem of optically resolving irregularities of such dimensions requires that the focus of the optical system be maintained so as to keep the diameter of the focussed spot to within a fraction of a micron.
One method of maintaining a constant disc-to-head spacing for an optical read head is described in U.S. Pat. No. 3,894,180, issued July 8, 1975 to Lawrence S. Canino. As described in the Canino patent, the reading of information recorded on a disc is through the use of a transducer head assembly placed adjacent the disc and which, along with the disc, is immersed in a fluid, (typically air) thus creating a fluid bearing between the assembly and the disc. As is further described in the Canino patent, the spacing maintained between the assembly and the disc can be kept substantially constant by means of a source of fluid at a variable controllable pressure, and a pressure regulator including a fluid flow restrictor which responds to changes in separation between the assembly and the disc and modifies the fluid pressure applied to the head.
Another Canino U.S. Pat. No. 4,006,294, issued Feb. 1, 1977 describes an improved transducer head assembly having a pair of expandable chambers which are coupled to each other through a fluid restrictor and which carries the transducer head, the expandable chambers being sensitive to pressure changes and causing the transducer head to move toward and away from the surface in response to such changes to correct the fluid flow pressure.
In both Canino patents, the forces exerted on the read head by the pressures within the chambers act directly on the entire resiliently suspended read head. This results in substantial mass and weight for such a read head reducing the ability of the head to respond to quick spacing changes between the head and the disc surface.
Other systems providing fluid bearings which afford a noncontact support for the entire disc and a read head and which utilize a negative pressure differential in the vicinity of the head may be found by a reference to U.S. Pat. No. 3,914,541, issued Oct. 21, 1975 to James E. Elliott, and to U.S. Pat. No. 3,947,888, issued Mar. 30, 1976 to Manfred H. Jarsen. In both of these patents, the read head is maintained substantially "rigid", while the disc is flexible and deemed "compliant" in maintaining a predetermined spacing between a disc surface and the read head. In such a system, the spacing is maintained by the creation of a hydrodynamic bearing between the videodisc and the foot of the read head. In both the Elliott and Jarsen patents, however, all forces acting to maintain the spacing constant are applied as forces to or are forces which react with the entire read head assembly. Furthermore, such prior art devices are useful only in systems employing "floppy" discs and would not be suitable for recording and/or playing back rigid discs, as in a disc mastering machine which records information on a coated substrate of thick glass. Both Elliott and Jarsen make reference to the need to assure appropriate spacing when dealing with the rigid videodisc or when dealing with a rigid master disc by applying various biasing forces brought to bear upon the read head assembly, urging it toward the surface of the disc.
In one such prior art apparatus, a positive pressure is applied to an annular chamber adjacent one end of a lens and housing assembly in which the lens assembly is telescopically mounted relative to the housing, and the positive pressure within the annular chamber acts on a flange provided at the forward end of the lens assembly, the air pressure entering the annular chamber being regulated by an air pressure regulator to provide the necessary biasing forces urging the lens assembly toward the videodisc spinning beneath it at 1800 RPM. Such a system has proven useful when recording or reading rigid videodiscs. One of the contributions of such prior art devices lies in the separating of the read head into two telescoping parts, the lens assembly and the housing, thereby making the low mass lens assembly responsive to quick axial adjustments so as to better maintain a constant spacing, and therefore constant focus, of the light beam reaching the disc.
In all of the above-referenced patents describing the prior art, some discussion and details have been given as regards to a servo system which responds to out-of-focus conditions and adjusts the biasing forces on the head accordingly. Furthermore, these references teach the need for providing a fluid pressure system responsive to the relative radial location of the read head to vary the pressure differential with radial location thereby maintaining a constant head-to-disc spacing independent of the relative surface speeds.
The present invention concerns the improvement in the support of the lens assembly relative to the housing, and this description will not dwell on the pneumatic servo system and its cooperation with a radial position sensing device, and it is presumed that one skilled in the art can adapt such subsystems to a recorder or player apparatus incorporating the concepts of the present invention.
A major drawback in the prior art head assemblies incorporating the aforementioned annular chamber concerns the chattering, or lateral and axial vibrations, of the lens assembly relative to the housing due to the turbulent airflow in the passageway leading from the annular chamber to the atmosphere. The chattering is caused by the short axial length of the flange relative to the gap between the outer periphery of the flange and the inner periphery of the housing.
To determine the cause of the vibrations set up in the prior art heads, an accelerometer was installed on the lens assembly. It was discovered that excessive vibration in both the axial and radial directions occurred, and the source of the vibrating forces appeared to center around the gap location of the journal bearing. Using known techniques to observe the streamlines of air passing through the gap, it was discovered that a separation layer existed close to the piston wall, and a substantial air turbulence was noted downstream of the gap exit. Apparently, the separation layer attaches and separates itself from the piston wall randomly creating vibration by adding to and subtracting from the pressure of the air source from the regulator.
Attempts at diminishing the turbulent air flow have been directed at lengthening of the flange resulting in a lengthening of the lens assembly, thereby adding undesirable mass and increasing the length of the free end of the lens assembly such that housing support for the lens assembly is a greater distance from such free end causing instability and inaccuracies in maintaining solid lateral support for the lens assembly within the housing. Obviously, it is desirable to have the bearing surfaces between the lens assembly and the housing extend the entire length of the lens assembly, and thus there arises a trade-off between axial bearing length and axial flange length, the former needed for solid lateral support of the lens assembly and the latter needed for minimizing air turbulence which causes lateral and axial vibrations of the flange and the integral lens assembly.